Light emitting diode

ABSTRACT

A light emitting diode includes a substrate having a body of single crystalline semiconductor material, preferably a Group III-V compound or alloy thereof, on a surface of the substrate. The body includes a window layer directly on the substrate and one or more other layers on the window layer. The layers of the body are of appropriate conductivety types to form a recombination region in which light can be generated. The substrate has an opening therethrough to the window layer. The window layer is of a material which is substantially transparent to the light generated in the recombination region and is of a thickness, 15 to 30 microns, to provide rigidity to the diode.

BACKGROUND OF THE INVENTION

The present invention relates to a light emitting diode, and particularly to a surface emitting light emitting diode having good rigidity.

In the development of light emitting diodes (LEDs) for optical fiber communications, two basic types of diodes have emerged: edge emitting LEDs and surface emitting LEDs. An edge emitting LED in general includes a body of a semiconductor material, generally of a Group III-V compound or alloy thereof, having a recombination region therein in which the light is generated and which extends to an edge of the body. There is also included a waveguide which helps direct the light generated in the body to the edge of the body where the light can be coupled to an optical fiber.

A surface emitting LED in general includes a body of a semiconductor material having a recombination region therein close to a surface of the body, wherein the material of the body between the recombination region and the emitting surface is substantially transparent to the light. This transparent region is generally referred to as the "window". Thus, the light generated in the recombination region can pass through the window to the surface where it is coupled into an optical fiber.

One type of surface emitting LED has the body of the semiconductor material formed on a substrate which has an opening therethrough to the emitting surface of the body. Such an LED is shown in U.S. Pats. Nos. 4,053,914 by Anthony Richard Goodwin entitled "LIGHT EMISSIVE DIODE", issued Oct. 11, 1977; 4,010,483 by Yet-Zen Liu entitled "CURRENT CONFINING LIGHT EMITTING DIODE" issued Mar. 1, 1977; and 3,936,855 by James Emanuel Goell et al entitled "LIGHT EMITTING DIODE FABRICATION PROCESS" issued Feb. 3, 1976. However, as pointed out in an article by J. C. Dyment et al entitled "PROTON BOMBARDMENT DOUBLE HETEROSTRUCTURE LEDS", published in Journal Electronic Materials, Vol. 6, No. 2, 1977, pp. 173-193, a problem with this type of LED is a high tendency for initial failure, i.e., failure after a very short period of operation.

SUMMARY OF THE INVENTION

A light emitting diode including a substrate, and a body of semiconductor material on the substrate. The body includes a window of a material which is transparent to the generated light and is of a thickness of between 15 microns and 30 microns and means capable of generating light. The substrate has an opening therethrough to the window.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE of the drawing is a cross-sectional view of one form of a light emitting diode incorporating the present invention.

DETAILED DESCRIPTION

Applicants have discovered what they believe to be the reason for the high tendency for initial failure in the type of surface emitting LEDs shown in U.S. Pat. Nos. 4,053,914, 4,010,483 and 3,936,855. They believe that it results from defects in the body of the LED. The windows of these devices are relatively thin, not greater than 10 microns and generally not greater than 2 microns in thickness, and the substrates are also relatively thin. The thin window is used to insure the transmission of the generated light therethrough. However, these thin dimensions make the LEDs more susceptible to damage during handling, resulting in the high tendency for initial failure. To overcome this problem, applicants have developed an LED which is strong, and therefore not as easily damaged, yet still provides good transmission of light from the recombination region to the emitting surface.

Referring to the drawing, one form of a light emitting diode (LED) which includes the present invention is generally designated as 10. The LED 10 includes a substrate 12 and a body 14 of a single crystal semiconductor material, preferably a Group III-V compound or alloy thereof, on a surface of the substrate 12. The substrate 12 is made of a material which will propagate the epitaxial growth of the material of the body 14. Thus, for a body 14 of a Group III-V compound or alloy thereof, the substrate 12 is preferably of a Group III-V compound, such as gallium arsenide. The substrate 12 is of a conductivity type which is the same as that of the adjacent portion of the body 14 and contains sufficient conductivity modifier to be highly conductive. Thus, if the adjacent portion of the body 14 is of N type conductivity, the substrate 12 is of N+ type conductivity. The substrate 12 has an opening 16 therethrough to the surface 18 of the body 14 and may have a thickness of up to about 70 microns.

The body 14 includes a window layer 20 on the surface 13 of the substrate 12. The window layer 20 is of N type Al_(x) Ga_(1-x) As, where x varies from about 0.4 at the substrate surface to about 0.2 at its other surface, has a thickness of between about 15 and 30 microns, preferably between 20 and 25 microns.

On the window layer 20 is a layer 22 of P type Al_(y) Ga_(1-y) As, where y is between 0.05 and 0.08. The layer 22 has a thickness of about 2 to 2.5 microns and is the recombination region of the LED. On the recombination region layer 22 is a thin layer 24 of P type Al_(z) Ga_(1-z) As, where z is about 0.2 to 0.25. The layer 24 has a thickness of about 1 to 1.5 microns. On the layer 24 is a contact layer 26 of P+ type gallium arsenide having a thickness of about 0.3 to 0.5 micron. On the contact layer 26 is a thin insulating layer 28, preferably of a silicon dioxide. This insulating layer 28 has an opening 30 therethrough to the surface of the contact layer 26. A metal contact film 32 is on the insulating layer 28 and extends into and across the opening 30 to make contact with the contact layer 26. A second metal contact film 34 is on the exposed surface 15 of the substrate 12. The contact film 32 on the P+ contact layer 26 may be made of any metal which adheres well to and has good ohmic contact with the semiconductor material. For example, a trilayer of first titanium, then platinum, then finally gold, has been found to be satisfactory for the contact film. The contact film 34 to the N substrate 12 may be of tin, covered with nickle and then gold.

The LED 10 is made by depositing the various layers of the body 14 on the substrate 12 using the well known technique of liquid phase epitaxy. This may be achieved using the apparatus and method described in U.S. Pat. No. 3,753,801 to H. F. Lockwood et al, entitled "METHOD OF DEPOSITING EPITAXIAL SEMICONDUCTOR LAYERS FROM THE LIQUID PHASE", issued Aug. 21, 1973. The apparatus includes a furnace boat having a plurality of wells in the top surface thereof (one well for each of the layers to be deposited), a slide movable through a passage extending across the bottoms of the wells, and a weight in each of the wells. The slide has a pair of recesses in its top surface, one for receiving a source wafer and the other for receiving the wafer which will form the substrate of the LED. The recess for receiving the substrate wafer is of a depth with respect to the thickness of the wafer such that the top surface of the substrate wafer is spaced from the top of the recess a distance at least equal to the thickness of the body 14 to be formed plus some clearance, about 100 microns. For making the LED 10, the furnace boat has five wells therein, one for each layer of the body 14 and one for use in smoothing out the surface of the substrate.

In each of the wells is placed a charge containing the semiconductor material to be deposited, a solvent for the semiconductor material, and a conductivity modifier. For making the LED 10, gallium is used as the solvent, tin is used as the N type conductivity modifier and germanium as the P type conductivity modifier. In the first well is placed 340 mg of gallium arsenide, 3000 mg of gallium and 400 mg of tin. In the second well which forms the window layer 20, there is provided a charge of 200 mg of gallium arsenide, 4.4 mg of aluminum, 3000 mg of gallium, and 300 mg of tin. In the third well to form the recombination region layer 22, there is provided a charge of 300 mg of gallium arsenide, 1.4 mg of aluminum, 3000 mg of gallium and 10 mg of germanium. In the fourth well to form the layer 24, there is provided a charge of 200 mg of gallium arsenide, 4.7 mg of aluminum, 3000 mg of gallium and 100 mg of germanium. In the fifth well to form the contact layer 26, there is provided a charge of 340 mg of gallium arsenide, 3000 mg of gallium and 300 mg of germanium.

A source wafer of gallium arsenide is placed in the first recess in the slider to be used to bring each of the solutions to saturation. The gallium arsenide substrate wafer is placed in the second recess. A weight is placed in each of the wells over the charge.

The furnace boat and its contents are placed in a furnace and a flow of high purity hydrogen is provided through the furnace and over the furnace boat. The heater for the furnace is turned on and the furnace is raised to a temperature of about 920° C. and left at that temperature for a period long enough to achieve melting and homogenizing of the ingredients of the charges in the wells of the furnace boat, about three to five hours. The furnace is then cooled to bring the furnace boat and its contents to a starting temperature of about 890° C. The slide is then moved to bring the substrate wafer into the first well where it contacts the solution therein. The temperature of the furnace boat is then raised 1 to 2 degrees centigrade to cause a slight melting back of the surface of the substrate wafer into the solution in the first well. The temperature of the furnace boat is then dropped 2 to 3 degrees centigrate to cause some of the gallium arsenide in the solution in the first well to precipitate out and deposit on the surface of the substrate wafer to provide the substrate wafer with a smooth surface.

The slide is then moved to bring the substrate wafer into the second well. The furnace boat and its contents are then cooled between 32° to 40° at a rate of about 0.64° C. per minute. This causes some of the aluminum gallium arsenide in the second well to precipitate out and deposit as the window layer 20 on the substrate wafer. Some of the tin will be incorporated in the aluminum gallium arsenide so that the deposited layer will be of N type conductivity. Depositing this layer over this long period of time will provide a layer which is of a thickness of between 20 and 25 microns. When the layer begins to deposit the aluminum gallium arsenide will have a high content of aluminum, about 33 atomic %. However, as the deposited layer builds up, the concentration of aluminum will decrease to about 20 atomic % because of the depletion of the aluminum from the solution, as well as the difference in segregation coefficients between aluminum and arsenic.

The slide is then moved into the third well and the temperature lowered between 2° and 2.4° C. to cause some of the gallium aluminium arsenide in the solution to precipitate out and deposit as the recombination region 22. Some of the germanium in the solution will deposit with the aluminum gallium arsenide to form a P type conductivity layer. This layer will have a thickness of between 2 and 2.5 microns.

The slide is then moved to carry the substrate wafer into the fourth well and the furnace boat and its contents are cooled between 1.7° and 2° C. This will cause some of the gallium aluminum arsenide in the solution in the fourth well to precipitate out and deposit as the layer 24 having a thickness of between about 1.2 and 1.8 microns. The slide is then moved to carry the substrate wafer into the fifth well where the temperature is reduced between 0.15 and 0.2 degrees centigrade. This causes some of the gallium arsenide in the solution in the fifth well to precipitate out to form the contact layer 26. Some of the germanium in the solution will deposit with the gallium arsenide to form a P type layer. Since the solution in the fifth well contains a large amount of germanium, the deposited layer will have a high concentration of the germanium which results in a low resistivity.

The slide is then moved to carry the substrate wafer out of the fifth well and the furnace is then cooled sufficiently to permit removal of the coated substrate from the furnace boat. During the above described deposition steps, the source wafer in the first recess in the slide is moved into each of the wells prior to the substrate wafer so as to saturate each of the solutions prior to bringing the substrate wafer into contact with the solutions. This provides for better control of the layers deposited from the solutions.

The insulating layer 28 is coated on the contact layer 26 by the well known technique of vapor deposition. The opening 30 is formed in the insulating layer 28 by providing a resist material over the insulating layer 28, except where the opening is to be formed, and then etching away the exposed portion of the insulating layer with a suitable etchant. The metal contact layers 32 and 34 may be coated on the LED 10 by any well known technique such as vacuum evaporation. Alloying of the layer 32 by heating at 400° C. in hydrogen for a short time may be necessary.

The opening 16 in the substrate 12 may be formed by etching into the substrate 12 with a mixture of 95% by volume of H₂ O₂ (30% solution) and 5% by volume of NH₄ OH (29% solution of NH₃) which will stop etching at the surface of the N type aluminum gallium arsenide window layer 20.

In the operation of LED 10, the light is generated in the recombination region layer 22. By limiting the area of contact between the contact film 32 and the contact layer 26 the flow of electrons across the body 14 is confined so that the light is generated in a confined portion of the recombination layer 22 which is aligned with opening 16 in the substrate 12. The light passes through the window layer 20, which is transparent to the light, to the surface 18. An optical fiber can be inserted into the opening 16 on the substrate 12 to couple the light into the optical fiber.

The LED 10 has a thick window layer 20 so that the LED is more sturdy. Therefore, the LED 10 is less susceptible to being damaged during handling which results in A LED which is more reliable, particularly with regard to initial failure. Although the window layer 20 is considerably thicker than the window layer of the prior art LEDs, we have found that contrary to what would be expected, the external efficiency, i.e., the amount of light transmitted through the window, is not affected since there is only negligible absorption of light in the window layers even though the window layer is thicker. In addition we have found that the thicker window layer can be easily deposited on the substrate in a single step by liquid phase epitaxy so that the thicker window layer does not substantially increase the cost of making the LED. We have found that a window layer of a thickness between 15 and 30 microns provides the desired rigidity without adversely affecting the external efficiency of the LED or the manner of making the LED. A window layer of a thickness less than 15 microns will be too thin to provide the desired rigidity and above 30 microns will be too difficult to deposit in a single step process. 

We claim:
 1. A light emitting semiconductor device comprisinga body of semiconductor material on a substrate of a semiconductor material, said body including a window layer adjacent to said substrate, and layers of opposite conductivity type on said window layer with the layer adjacent said window layer being a recombination region in which light can be generated, said window layer being of a material which is transparent to said generated light and having a thickness of between 15 and 30 microns, and said substrate having an opening therethrough to said window layer.
 2. A light emitting semiconductor device in accordance with claim 1 wherein the body is of a Group III-V compound or alloy thereof.
 3. A light emitting semiconductor device in accordance with claim 2 wherein the window layer is of Al_(x) Ga_(1-x) As where x is about 0.4 at the substrate and decreases across the thickness of the window layer to about 0.2.
 4. A light emitting semiconductor device in accordance with claim 3 wherein the substrate is of a material which will propagate the epitaxial growth thereon of the semiconductor material of the window layer.
 5. A light emitting semiconductor device in accordance with claim 4 in which the substrate is gallium arsenide.
 6. A light emitting semiconductor device in accordance with claim 5 wherein the substrate and the window layers are of the same conductivity type. 